1. Field of the Invention
The present invention relates to a polymerization liquid for electrolytic polymerization that can rapidly produce a conductive polymer layer with high conductivity and heat resistance. The present invention also relates to a conductive polymer film and a solid electrolytic capacitor which are obtained from the polymerization liquid.
2. Description of the Related Art
A conductive polymer obtained from polymerization of a monomer with a π-conjugated double bond such as thiophene, aniline and pyrrole is highly conductive and suitable for various electrochemical purposes, and has been applied to various fields such as a solid electrolytic capacitor as well as a polymer battery, an antistatic film, an indicating element, a sensor, and an electrode material. For example, the conductive polymer is suitably used in a solid electrolytic capacitor that contains a positive electrode in which an oxide film as a dielectric is provided on the surface of a valve metal foil such as aluminum, tantalum or niobium and a conductive polymer layer that contacts the oxide film and functions as a true negative electrode.
These conductive polymers can be obtained by an electrolytic polymerization method or a chemical polymerization method (for example, see Patent Document 1 (JP H1-313521 A)), and by the electrolytic polymerization method, whereby a conductive polymer film with excellent mechanical strength is formed on an electrode from a small quantity of monomer in a short period of time. The conductive polymer formed on the electrode is used in a form where it is separated from the electrode or in a form where it is kept placed on the electrode.
A polymerization liquid for electrolytic polymerization that is used to obtain these conductive polymers generally contains a monomer with π-conjugated double bond, a supporting electrode, and a solvent to dissolve them. The solvent of the polymerization liquid is suitably selected based on a consideration of the solubility of a monomer or other factors.
For example, Patent Document 1 discloses a method to obtain a conductive polymer by electrolytic polymerization of a 3,4-disubstituted thiophene such as 3,4-ethylenedioxythiophene (hereinafter 3,4-ethylenedioxythiophene is referred to as “EDOT” and poly(3,4-ethylenedioxythiophene) is referred to as “PEDOT”), and in working examples, acetonitrile is used as a solvent of a polymerization liquid. Patent Document 2 (JP S61-239617 A) discloses a method to obtain a conductive polymer by electrolytic polymerization of an aniline substituted by an alkyl group or an alkoxyl group, and in working examples, water is used as a solvent of the polymerization liquid.
Patent Document 3 (JP H3-18009 A) discloses that electrolytic polymerization using polymerization liquid containing non-substituted pyrrole as a monomer and p-toluenesulfonate as a supporting electrolyte forms a conductive polymer layer of a solid electrolytic capacitor, and in working examples, acetonitrile or water is used as a solvent of the polymerization liquid. Patent Document 4 (JP H2-58818 A) discloses that electrolytic polymerization using a polymerization liquid containing an organic solvent, a monomer such as pyrrole, thiophene or azulene, and an alkyl-substituted ammonium borodisalicylate as a supporting electrolyte forms a conductive polymer layer of a solid electrolytic capacitor. Patent Document 4 also states that a polymerized film with a problem in terms of thermal stability is obtained from a polymerization liquid where ammonium borodisalicylate with low solubility against organic solvent is used as a supporting electrolyte and a solvent in which water is added to an organic solvent is used.
As can be understood from these patent documents, both water and an organic solvent can be suitably selected and used as a solvent of a polymerization liquid for electrolytic polymerization. However, compared with usage of water solvent, usage of organic solvent generally increases the environmental burden and is economically disadvantageous. Further, many organic solvents are hazardous to humans, and in the case of electrolytic polymerization using a combustible solvent, it is necessary to take preventive measures against fire caused by an electric spark. Because the aniline derivative and non-substituted pyrrole specified in Patent Document 2 and Patent Document 3 have comparatively high solubility in water, water can be used as a solvent for polymerization of these monomers, but for polymerization of a monomer that is insoluble or slightly soluble such as the thiophene derivative mentioned in Patent Document 1, there was no other option but to use an organic solvent.
Therefore, also in the case of polymerization of a monomer that is insoluble or slightly soluble in water, a polymerization liquid has been considered in which an anion surfactant or a nonionic surfactant is used and the monomer is emulsified with water, in order to use water as a solvent having a small environmental burden and excellent economic efficiency.
For example, Patent Document 5 (JP 2000-269087 A) reports electrolytic polymerization using an aqueous polymerization liquid where a thiophene derivative such as EDOT is emulsified by an alkylnaphthalenesulfonate surfactant. Because an alkylnaphthalenesulfonate anion that is taken in a conductive polymer layer as a dopant is bulky, de-doping is inhibited, and a conductive polymer layer that is stable at a high temperature in high humidity is obtained. Also, Non-Patent Document 1 (Synthetic Met. (2009), 159 (5-6), 406-414) reports electrolytic polymerization using an aqueous polymerization liquid in which EDOT and sodium polystyrene sulfonate as an anionic surfactant are dissolved. By the action of sodium polystyrene sulfonate, solubility of EDOT in water is increased, EDOT becomes easily oxidizable, and a uniform film is formed on a Pt electrode.
There is no problem if an anion produced by an anion surfactant is used as a dopant of a conductive polymer, but a problem occurs if other anions are intended to be a dopant for the conductive polymer. If an anion surfactant and a supporting electrolyte from which a dopant is produced in a polymerization liquid are used concurrently, the anion surfactant inhibits the anion of the supporting electrolyte from being taken in the conductive polymer layer as a dopant, and the characteristics of the conductive polymer layer obtained such as conductivity and heat resistance are affected.
The applicants reported in Patent Document 6 (WO2011/108254A1) and Patent Document 7 (WO2011/108255A1) as well as in PCT/JP2012/55284 and PCT/JP2012/55285, which were not published at the time of this application, that electrolytic polymerization using an aqueous polymerization fluid containing borodisalicylic salt and bis(pentafluoroethanesulfonyl) imidate as a supporting electrolyte produces a conductive polymer with more excellent heat resistance compared with electrolytic polymerization using a polymerization liquid containing alkylnaphthalenesulfonate and polystyrene sulfonate, which produce a dopant that is bulky and has difficulty causing de-doping. Patent Document 4 states that a polymerized film with a problem in terms of thermal stability is obtained from a polymerization liquid where a solvent in which water is added to an organic solvent is used, but by using the suitable supporting electrolyte, a conductive polymer layer with excellent thermal stability can be obtained from the aqueous polymerization fluid. However, if this preferable supporting electrolyte and an anion surfactant are used concurrently in a polymerization liquid, it becomes difficult for the anion of the suitable supporting electrolyte to be taken into the conductive polymer layer as a dopant, so the heat resistance of the conductive polymer layer is lowered.
On the other hand, if a polymerization liquid containing a supporting electrolyte and a nonionic surfactant is used, the nonionic surfactant is not taken into a conductive polymer layer as a dopant and the taking-in of the anion of the supporting electrolyte into the conductive polymer as a dopant is not inhibited by the nonionic surfactant, because the nonionic surfactant is not ionized. Patent Document 8 (JP H2-235321 A) takes note of this point and discloses a manufacturing method of a solid electrolytic capacitor using a polymerization liquid containing a nonionic surfactant. By using a polymerization liquid in which a monomer for a conductive polymer, an anion as a dopant, and a nonionic surfactant such as polyethylene glycol, polyethylene glycol alkyl ethers, polyethylene glycol fatty acid esters or fatty acid monoglycerides are dissolved and electrolyzing by using an anode foil on which a dielectric oxide film is formed as an anode, a flat and smooth conductive polymer layer that closely contacts the dielectric oxide film is obtained.
Also, Patent Document 9 (JP 2008-37975 A) discloses a method to manufacture a conductive polymer by using a polymerization liquid, which contains a monomer for a conductive polymer, a supporting electrolyte, and a nonionic surfactant selected from compounds having a polyoxyalkylene group such as a (poly)styryl phenol-type polyoxyalkylene compound, a (poly)styryl phenol-formaldehyde condensate-type polyoxyalkylene compound or a (poly)alkyl-substituted phenol-formaldehyde condensate-type polyoxyalkylene compound, in an electrolytic polymerization reaction. By using a specific range of nonionic surfactant with a polyoxyalkylene group, a polymerization liquid with excellent stability is obtained, and a conductive polymer film with excellent evenness and conductivity is obtained. Patent Document 10 (JP 2008-118060 A) discloses a method to obtain a solid electrolytic layer of a solid electrolytic capacitor by electrolytic polymerization using a polymerization liquid containing a monomer for a conductive polymer, an aromatic sulfonate ion as a dopant, and the same nonionic surfactant as is disclosed in Patent Document 9. By using the polymerization liquid, a solid electrolytic capacitor with an even, dense, solid electrolytic layer, which has low equivalent series resistance (ESR) and leakage current, is obtained.